1. Field of the Invention
The present invention relates to a collet chuck assembly for holding a tool or workpiece on a spindle of a lathe.
2. Description of Related Art
Collet chucks are used to clamp or grip workpieces or tools in turning machines, such as lathes. The collet head inside the chuck includes a number of circumferentially-spaced gripping segments and a number of cam surfaces. When the collet head is moved axially, its cam surfaces interact with corresponding opposing surfaces or cams on the mounting fixture. The interaction of the opposing cam surfaces causes the gripping segments of the collet head to expand or contract, thus causing it to grip or release a workpiece or tool, depending on the direction of axial movement.
Collet chucks are commonly used in place of other types of chucks, especially for smaller workpieces, because they are typically more accurate and can transmit more torque than a conventional chuck, such as a jaw chuck. Collet chucks generally maintain their grip at high rotational speeds, whereas the centrifugal forces present at high rotational speeds may cause a conventional jaw chuck to loosen its grip on the workpiece.
In modern multi-spindle lathes, pick-off spindles are used in addition to the main spindle or spindles. After work on a part is completed on the main spindle, the pick-off spindle is used to remove the part from the main spindle and hold it so that it can be cut from the bar stock from which it is formed. Once the part is cut from the bar stock, the pick-off spindle holds the part while machining or finishing work is performed on the cut end. A multi-spindle lathe of this type and a typical process for employing a pick-off spindle are disclosed in U.S. Pat. No. 5,207,135, the contents of which are incorporated by reference herein in their entirety.
Typically, pick-off spindle collet heads are designed to grip the outer diameter (OD) of the workpiece by contracting to grip the workpiece and expanding to release it. A forward motion of the pick-off spindle nose is usually used to close the collet head around a workpiece, while a rearward motion of the pick-off spindle nose is used to open the collet head. One disadvantage of these typical pick-off spindle collets is that OD gripping limits the area of the workpiece that may be machined, because the area being gripped by the collet head cannot be machined.
Inner diameter (ID) gripping collets that grip a workpiece along an inner circumferential surface have also been used with the pick-off spindles of multi-spindle lathes. However, conventional ID gripping collets may be inconvenient to use because these collets typically require a rearward motion of the pick-off spindle nose in order to expand and grip a workpiece, the opposite motion required of an OD gripping collet.